transfer switch and shared neutrals

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rsh_757

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Bear with me as I strictly do commercial work and this is a residential question. I have been told by a co-worker that when installing a manual transfer switch that if a circuit with a shared neutral is connected to the switch, that the remaining circuit that uses the shared neutral must be connected to the transfer switch also.

First off, I cannot find anything that states that, I have looked in the 700's and 210 where the shared neutral itself is addressed.

Second, what is the point? I am trying to find out for sure before I question him and ask him to show me.

Thanks!
 
Would this "residential question" have to do with your own residence? Your profile tells me you are not an electrician. Members of this forum are not permitted to assist a person who is not an electrician, and who intends to perform electrical installation work in their own home.
 
It is not a code rule so much as the design of many residential transfer switches.

Many are nothing like what we deal with in commercial.

Here is one...

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Circuit 'C' & 'D' can be used for a 240 load or a multiwire branch circuit.
 
Its an interesting question. What happens in the case where you have both utility power and generator power at the same time, and you put only one side of a multiwire circuit on the transfer switch (and assuming a solid neutral system where none of it is switched and is just a giant bus)?

Wouldn't the neutral be carrying the load of both sources, but there is no phase relationship between those sources. So I don't see the currents subtracting, I would think they would add.

I don't know of anything in the code that would mandate that both sides of a multiwire be terminated to breakers on the same system. Perhaps it is no longer a multiwire circuit if you can't ensure the phase relationship between the two ungrounded conductors so you can't share it.

Seems like its a good idea to me to put them both on the switch, or minimize the time that a multiwire circuit is running on both utility and generator power at the same time.
 
How would both sources be connected at the same time to make this scenario possible?

suemarkp said:
Wouldn't the neutral be carrying the load of both sources, but there is no phase relationship between those sources. So I don't see the currents subtracting, I would think they would add.
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No the circuits would trip each other assuming they were on the sam esize breaker. The neutral of one would not be seen as a neutral to the second circuit since they are different systems. It would almost be a phase to phase short even though the phases are not of the same system.

How would both sources be connected at the same time to make this scenario possible?
 
charlie b said:
Would this "residential question" have to do with your own residence? Your profile tells me you are not an electrician. Members of this forum are not permitted to assist a person who is not an electrician, and who intends to perform electrical installation work in their own home.
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Negative. This is to answer a question that was posed by another co-worker for his weekend work. I worked commercial for 5 years for a large aerospace company with backup systems that aren't even close to what one's residence would use. I still work for that aerospace company but now I don't do any electrical work. The subject came up while having coffee on a break as I still work with that crew, just not "with" that crew. Funny thing is, the question had at least 6 experienced electricians arguing one way or another, but no one could find reference to it in NEC. Personally, I just want to see what some others think.
 
Because the power came back on and the generator has not been turned off yet (or someone is testing the generator during periodic maintenance). So the utility panel and the generator are both on. These little transfer switches can only transfer 6 to 10 loads. All the others in the main panel are still powered when the utility service is back on.

One leg of the multiwire circuit is switched over to the transfer switch. The other leg does not go to the transfer switch, so it is always only on utility power.

I still think the neutral currents have to be additive because they are of different systems.

A similar analogy would be a 120V circuit and a 12VDC circuit using a 12-3 cable. The - of the battery shares the neutral with the grounded portion of the AC circuit. The ungrounded AC conductor gets the black and the 12 VDC + gets the red wire. If both the AC and DC circuits draw 20A, what is the effective heating on the neutral -- 20A worth or 40A?
 
suemarkp said:
A similar analogy would be a 120V circuit and a 12VDC circuit using a 12-3 cable. The - of the battery shares the neutral with the grounded portion of the AC circuit. The ungrounded AC conductor gets the black and the 12 VDC + gets the red wire. If both the AC and DC circuits draw 20A, what is the effective heating on the neutral -- 20A worth or 40A?
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What?:confused: ? When would this situation ever occur??

210.23 says that the load shall not exceed the branch circuit rating, which is what would happen to the neutral of a multi-wire circuit in any other configuration than running it with two opposite phases of the same system. (Utility or Generator, but not both at the same time)
 
rlMutch said:
What?:confused: ? When would this situation ever occur??

210.23 says that the load shall not exceed the branch circuit rating, which is what would happen to the neutral of a multi-wire circuit in any other configuration than running it with two opposite phases of the same system. (Utility or Generator, but not both at the same time)
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The above situation would not, I was just making a comparison to two separate systems using a common wire. I think the possibility of a generator being on and the utility being on at the same time is very possible. This gets back to the original question asked -- must you put both legs of a multiwire branch circuit to a transfer switch if you only really wanted one leg there.

210.23 says yes, but I think it isn't obvious for a multiwire branch circuit. An electrician should get it right when the transfer switch is totally in Utility mode (he knows to put each ungrounded conductor on opposite legs). But allowing only one leg to run from the generator (while fine when only the generator is running) allows overloading if both utility and generator are active at the same time.
 
suemarkp said:
I don't know of anything in the code that would mandate that both sides of a multiwire be terminated to breakers on the same system.
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Mark, Check 2005 210.4(A) last sentence, it addresses it directly.

Then even if we forget about 210.4(A) we would probably run into issues with 300.3(B).
 
suemarkp said:
A similar analogy would be a 120V circuit and a 12VDC circuit using a 12-3 cable. The - of the battery shares the neutral with the grounded portion of the AC circuit. The ungrounded AC conductor gets the black and the 12 VDC + gets the red wire. If both the AC and DC circuits draw 20A, what is the effective heating on the neutral -- 20A worth or 40A?
Click to expand...

Neither 20A or 40A. You would be mixing 2 different wave forms. When you say DC would you be talking about flat line or filtered DC? or just pulsating DC? Imagine what the sum of the two wave forms would be. A 60 cycle AC sine wave plus whatever DC you add to it???? It would double the current. I don't even see how this would be an analogy because this would never happen.
 
This is the way I see this scenario and if someone believes I'm wrong please say so and explain why.

The manual transfer switch that Bob (Iwire) has shown is primarily used for installations where there is a portable generator which gets connected to this transfer switch via a cord and plug. This xfer switch is attached to a main breaker panel via the greenfield whip on the bottom of the unit. Inside the greenfield whip is a #8 green ground bonding wire and usually a #8 or #6 white neutral wire (depending on the size of the loads anticipated). In addition there are two wires per circuit that are connected in series between specific circuit breakers in the main panel and the respective transfer switch in the xfer switch panel. There is no specific 240 volt power feed from the main breaker panel to this xfer switch and therefore it is not considered a sub-panel even though the little white switches you see at the top of the xfer box are circuit breakers. All the branch circuits originate in the main breaker panel as do the neutrals. That means all the neutrals and EGC's are bonded together in the main breaker panel and are not relocated to another panel. It is quite possible that you could end up with all 120 volt loads from the same phase depending on where they were originally landed in the main breaker panel. ( Note :The double pole xfer switch can be converted to single pole simply by removing the handle tie). If this is the type of transfer switch you are using then you do not have to transfer the 2nd phase of the multi-wire branch circuit because you haven't relocated any neutral wires.

On the other hand, if you have a stationary generator with a combination sub-panel and automatic transfer switch then the neutrals must originate in the panel where the branch circuit is fed from. The reason is that the xfer switch/sub-panel is being fed from the main breaker panel via a (say 50 amp) 2-pole circuit breaker. The neutral in the sub-panel floats and is not bonded to the cabinet and EGC as in a main breaker panel. Inside the greenfield whip for this unit are 3 # 6 's (1-red, 1-black and 1 white) for the power connection, one #8 for the EGC and a black and white for each 120 volt branch circuit. You would have to remove the wire from a chosen breaker in the main panel and splice it to the respective black coming from the xfer switch panel. In addition you would have to find the corresponding neutral in the main panel and splice it to the respective neutral in the xfer switch panel. Once you do this the second phase of the multi-wire branch circuit in your scenario would have no neutral reference and would have to be transferred to an opposing phase in the sub-panel as well.

Didn't mean to be so long winded.

Phil,
Gold Star Electric
New Jersey
 
Once you do this the second phase of the multi-wire branch circuit in your scenario would have no neutral reference and would have to be transferred to an opposing phase in the sub-panel as well.
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I just wanted to correct this mis-statement I made. There would be a reference to neutral on the second half of this multi-wire branch circuit but it would be coming from the sub-panel instead of the main panel. While electrically speaking this circuit would still function I believe it is a code violation (210.4(A) All conductors must originate from the same panelboard).
 
Mike,
No the circuits would trip each other assuming they were on the same size breaker. The neutral of one would not be seen as a neutral to the second circuit since they are different systems. It would almost be a phase to phase short even though the phases are not of the same system.
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I don't see any reason that using a common neutral for circuits that originate in a generator and the utility would cause any breaker trips on either system. They are independent systems and there is no path between the two systems that would flow current to trip one or both of the breakers.
A 60 cycle AC sine wave plus whatever DC you add to it???? It would double the current. I don't even see how this would be an analogy because this would never happen.
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I wouldn't say never. I have seen a common conductor used in a jail locking system because the installer did not pull enough wires. The locking system used both 24 VAC and 24VDC. They were short one wire so they tied one side of both power supplies together and used that between the control panel and the doors. Not exactly the same, but I am sure that somewhere someone has used a common conductor like this with systems of higher power and voltage. I am not saying this a good idea, just that you really can never say never when the ingenuity of electricians is involved.
Don
 
Iwire said:
Guys I am lost, I am not sure about the 'reference' your talking about and regardless of the neutral 210.4(A) applies.
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Sorry Bob. Maybe I didn't explain myself clearly. In the case of a portable gen-set the neutrals never leave the main breaker panel. In addition the branch circuit is also fed from the main breaker panel so you would not be in violation of 210.4(A). In the case of a stationary gen-set with a combination breaker panel/xfer switch you would have to move both phases of a multi-wire branch circuit to the xfer panel or you would be in violation of 210.4(A)

I didn't get involved in the AC/DC portion of this thread.
 
Bob, it's OK to disagree but you should explain why. If you think about it you're not transferring neutrals in the xfer switch panel so it's not an SDS. All the EGC's and neutrals (including the neutral from the generator) are bonded together in the MB panel. How could this be in voilation of 210.4(A) ? Am I not thinking "out of the box" again ?
 
Phil, with the utility on and the generator running you are supplying the MWBC from two different panels or distribution equipment. A violation of 210.4(A)

You know that will happen each time the power comes on after a power failure.
 
Bob,

I guess the problem I'm having with this is that the xfer panel you've shown in your post is similar to one made by Generac. There are no provisions in that panel for individual branch circuit neutrals. There is one #6 that supplies the neutral from the generator and gets terminated to the neutral buss bar in the main breaker panel where all neutrals and EGC's are bonded together to earth ground. I can't see how there could be a possibility for feed-back through the neutral buss from another circuit (or power source). So, whether we're talking about MWBC's or not, once you transfer any branch circuit from utility power to generator power, by your interpretation, it would be a violation of 210.4(A). If this is the case then the design of this entire manual xfer switch is in violation and should not be manufactured.
 
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